AU2012257925B2

AU2012257925B2 - Communication system and corresponding method, computer program, and storage means

Info

Publication number: AU2012257925B2
Application number: AU2012257925A
Authority: AU
Inventors: Geraud Allard; Yves Bard; Patrice Courtet; Gilles Raymond
Original assignee: Sagem Defense Securite SA
Current assignee: Safran Electronics and Defense SAS
Priority date: 2011-05-17
Filing date: 2012-05-02
Publication date: 2016-03-03
Anticipated expiration: 2032-05-02
Also published as: FR2975560B1; EP2710820A1; CA2835257A1; FR2975560A1; EP2710820B1; WO2012156202A1; RU2013150025A; RU2595546C2; BR112013029227A2

Abstract

The invention relates to a communication system comprising communication nodes communicating via multi-frequency, time division multiple access frames. The nodes form a hierarchical cellular radio network that defines hierarchical levels which each include a base and at least one handset. The communication nodes are enabled to communicate via the base of the hierarchical level thereof. A master node comprises: a means (4.2) for selecting, in each of the frames, a transmission resource that is common to said communication nodes; a means (4.3) for broadcasting to the communication nodes, through the hierarchical levels via the respective bases thereof, information representative of the selected common transmission resource. The system is suitable for enabling said communication nodes to broadcast messages via the selected common transmission resource.

Description

Communication system and method, computer program and corresponding storage means The present invention concerns the field of cellular radio communication systems and more particular communication systems based on time division and frequency division multiple accesses. Cellular radio communication systems enable devices, referred to as handsets or 5 mobile terminals, to communicate with each other via at least one other device, referred to as base. A device, whether it be a handset or a base, is more generally referred to as communication node. Each base manages a cell and periodically sends a signal, referred to as beacon, which enables the handsets present in the coverage area of the cell to synchronise with 10 the base in order to establish communications and exchange information for managing the communication system. Such known radio communication systems use a time division multiple access (TDMA) mechanism. In these systems, the time is divided into frames serving to frame the communication. Each frame is itself divided into time intervals (time slots) 15 of predefined duration, in order to share the time for access to the transmission medium between the communication nodes. Such systems may also use a frequency division multiple access (FDMA) mechanism. For example, digital enhanced cordless telecommunication DECT (registered trade mark) systems, as specified in the series of documents ETS 300 175 published by the ETSI (European Telecommunications 5 Standards Institute), use transmission principles of the TDMA and FDMA type. Such TDMA and FDMA communication systems may be mobile. That is to say both the handsets and the bases may be mobile. In these mobile TDMA and FDMA communication systems, the transmission conditions between the communication nodes may change rapidly, for example by the appearance of a phenomenon of 10 jamming of the signal or by fading of the signal through the appearance of an obstacle between the communication nodes. The communication channels set up between the communication nodes may then suffer interference detrimental to the communication, or even become unusable. Although the cellular radio communication standards provide for an automatic 15 mechanism for transfer (handover) of a handset to another base, its setup is not sufficiently rapid to enable to counter so-called fast fading or fast shadowing phenomena. In addition, such fast fading or fast shadowing phenomena may be detected only by a subset of the communication nodes. Such a situation must be notified to the rest 20 of the communication system, so that a redefinition of the communication channels and the base-handset roles can be implemented as quickly as possible. The automatic handover mechanisms previously mentioned are not sufficiently reactive in this type of situation. It is desirable to overcome these various drawbacks of the prior art. 25 It is in particular desirable to provide a solution that improves the reactivity of communication systems composed of mobile communication nodes, more particularly in the context of transmission of TDMA and FDMA type, during rapid changes in transmission conditions. It is in particular desirable to provide a solution that improves the reactivity of 30 such communication systems when phenomena of signal jamming, fast fading or fast shadowing appear. It is in particular desirable to provide a solution that enables a handset to synchronise rapidly with a new base, when the communication with the base to which it was attached is broken.

It is in particular desirable to provide a solution that enables rapidly broadcasting, in such communication systems, information on change in transmission conditions. The invention concerns a communication system comprising communication 5 nodes communicating via time division and frequency division multiple access frames, said nodes forming a hierarchical cellular radio network defining hierarchical levels each comprising a base and at least one handset, said system being adapted so that the communication nodes are allowed to communicate via the base of their hierarchical level. The system is such that a communication node, referred to as 10 master node, comprises: means for selecting, in each of said frames, a transmission resource common to said communication nodes, referred to as transmission resource common to the network neighbourhood; means for broadcasting to said communication nodes, through the hierarchical levels via their respective bases, information representative of the selected common transmission resource. The system 15 is further adapted to allow said communication nodes to broadcast messages via the selected common transmission resource. Thus, by enabling the communication nodes to transmit data in a broadcast manner via the transmission resource common to the network neighbourhood, in addition to the communications via the base of the hierarchical level, the communication system is more reactive. 20 According to a particular embodiment, said master node comprises means for obtaining information representative of transmission conditions between said communication nodes, and the means for selecting the transmission resource common to the network neighbourhood are adapted to perform the selection according to said obtained information. Thus, the communication system is even more reactive when 25 phenomena of jamming, fast fading or fast shadowing appear. According to a particular embodiment, said communication nodes comprise means for determining transmission conditions in said communication system and means for transmitting messages containing information representative of transmission conditions, or change of transmission conditions either via the 30 transmission resource common to the network neighbourhood or via the base of their hierarchical level. According to a particular embodiment, said communication nodes comprise means for broadcasting, via the transmission resource common to the network neighbourhood, messages comprising synchronisation information identifying a 4 resource of said frames used for transmitting a beacon signal. Thus, a node for which the channel for communication with the base of its hierarchical level is broken can re insert itself in the system as quickly as possible. According to a particular embodiment, said communication nodes comprising 5 handset and base functions, said communication nodes comprise: means for detecting a risk of loss of connectivity with a communication node; means for activating a beacon signal, when a risk of loss of connectivity is detected; means for broadcasting, via the transmission resource common to the network neighbourhood, a message comprising synchronisation information identifying a resource in said frames, used for 10 transmitting said beacon signal. Thus, the connectivity of the communication system is enhanced. According to a particular embodiment, the base of each hierarchical level comprises means for enabling the communication nodes of its hierarchical level to broadcast messages via a transmission resource common to said hierarchical level. 15 According to a particular embodiment, the transmission resource common to each hierarchical level corresponds to a resource of said frames having a predefined position relative to a resource used by the base of said hierarchical level for transmitting a beacon signal. Thus, the resources usage is optimised. According to a particular embodiment, the base of each hierarchical level 20 comprises means for changing resource of said frames, used for transmitting a beacon signal, according to messages received via the transmission resource common to its hierarchical level or via the transmission resource common to the network neighbourhood. Thus, the communication system is more reactive. According to a particular embodiment, said system is adapted so that the bases 25 transmit their beacon signal in a time slot distinct from each time slot defining the transmission resource common to the network neighbourhood. The invention also concerns a communication method in a communication system comprising communication nodes communicating by means of frames of the time division multiple access type, said nodes forming a hierarchical cellular radio 30 network defining hierarchical levels each comprising a base and at least one handset, said system being adapted so that the communication nodes are allowed to communicate via the base of their hierarchical level. The method is such that a communication node, referred to as master node, performs steps consisting of: selecting in each of said frames a transmission resource common to said communication nodes, referred to as transmission resource common to the network neighbourhood; broadcasting to said communication nodes, through hierarchical levels via their respective bases, information representative of the selected common transmission resource. The method is further such that said communication nodes are 5 allowed to broadcast messages via the selected common transmission resource. The invention also concerns a computer program, which may be stored on a medium and/or downloaded from a communications network, in order to be read by a computer system or a processor. This computer program comprises instructions for implementing the method mentioned above, when said program is executed by the 10 computer system or the processor. The invention also concerns storage means comprising such a computer program. The features of the invention mentioned above, as well as others, will emerge more clearly from a reading of the following description of an example embodiment, said description being given in relation to the accompanying drawings, among which: 15 - Fig. 1 schematically illustrates a frame structure in which the invention can be implemented; - Fig. 2 schematically illustrates a communication system in which the invention can be implemented; - Fig. 3 schematically illustrates an architecture of a communication node in 20 the communication system of Fig. 2; - Fig. 4 schematically illustrates an algorithm for setting up a transmission resource common to the network neighbourhood; - Fig. 5 schematically illustrates a first algorithm for using the transmission resource common to the network neighbourhood; 25 - Fig. 6 schematically illustrates a second algorithm for using the transmission resource common to the network neighbourhood; - Fig. 7 schematically illustrates a resynchronisation algorithm using information broadcasted via the transmission resource common to the network neighbourhood; 30 - Fig. 8 schematically illustrates an algorithm for enhancing the connectivity of the network neighbourhood using information transmitted via the transmission resource common to the network neighbourhood. In order to increase the reactivity of TDMA communication systems to changes in transmission conditions, it is proposed to reserve, in each TDMA frame, a transmission resource common to the network neighbourhood, intended to enable each of the communication nodes to broadcast information in connectionless mode. This transmission resource common to the network neighbourhood is selected by a predefined node of the communication system, preferably according to the 5 transmission conditions, and is therefore, in this case, able to vary over time. This transmission resource common to the network neighbourhood thus enables every communication node to broadcast in the communication system information on synchronisation, topology management, network neighbourhood detection and/or detection of change in transmission conditions, independently of the base to which it 10 is attached. Network neighbourhood means the whole communication system, that is to say all the communication nodes allowed to communicate by means of the same frame. The setting up and use of this transmission resource common to the network neighbourhood in each TDMA frame are detailed hereafter in the illustrative context 15 of a DECT (registered trade mark) communication system. Fig. 1 schematically illustrates a frame structure 1.1 in which the invention can be implemented. The representation in Fig. 1 comprises two axes: on the vertical axis Y, various carrier frequencies are shown; and on the horizontal axis X, various consecutive time slots are shown. The representation in Fig. 1 illustrates for example a 20 frame according to the TDD (Time Division Duplex), TDMA and FDMA communication principles that are found in DECT (registered trade mark) communication systems. The frame 1.1 has a duration of 10 ms and is divided, on the time axis, into a first part 1.2 devoted to downlink traffic and a second part 1.3 devoted to uplink 25 traffic. Downlink traffic is sent by a base to at least one handset whereas uplink traffic is sent by a handset to a base. It is then said that the communications are half-duplex in that the downlink and uplink communications are not simultaneous but distributed over different time slots. In the representation in Fig. 1, the first 1.2 and second 1.3 parts are of the same 30 duration. Each of these parts is further divided into twelve elementary time slots of same duration. In the representation in Fig. 1, the frame 1.1 is also divided, on the frequency axis, into ten carrier frequencies. Thus, the frame 1.1 comprises elementary communication units 1.6 that respectively correspond to a carrier frequency and to a time slot. Each elementary 7 communication unit 1.6 is then defined by a unique frequency/time slot pair. The frame 1.1 shown in Fig. 1 thus comprises two hundred and forty elementary communication units 1.6. When two nodes communicate, a transmission channel is defined as a pair of 5 elementary communication units 1.6, one unit per communication direction. A first unit in the first part 1.2 of the frame 1.1 and a second unit in the second part 1.3 of the frame 1.1. Typically these elementary communication units 1.6 are corresponding, meaning that they use the same carrier frequency and are separated by twelve elementary time slots. Such is the case with the elementary communication units 1.6 10 marked with a cross in Fig. 1, meaning those that correspond to the ninth and twenty first time slots on the seventh carrier frequency. Each elementary communication unit 1.6 enables signals to be transmitted in the form of a data packet, which may be composed of a header field that contains signalling and protocol information and a payload data field that contains the useful 15 data exchanged. Typically, the payload data field contains the audio samples during a voice communication. To enable a handset to be connected to a base, this base sends a so-called beacon signal. This signal uses one of the elementary communication units 1.6 of the first part 1.2 of the frame 1.1. This signal is sent periodically, at each frame, and contains the 20 signalling and synchronisation information necessary for the handset to connect and synchronise with the base so as to be able to communicate with it. Typically the beacon signal sent by a base to enable handsets to be connected consists of signalling information contained in the header field of a data packet, and the payload data field is then meaningless. 25 Fig. 2 schematically illustrates a hierarchical communication system in which the invention may be implemented. Such a communication system may be intended for a set of individuals, at least some of whom are mobile, and who carry respectively on themselves communication devices 2.1, 2.2, 2.3, 2.4, 2.5 and 2.6, also referred to as communication nodes. The communications between nodes are performed by 30 means of frames having the format of the frame 1.1. Each of the communication nodes 2.1, 2.2, 2.3, 2.4, 2.5 and 2.6 preferably has handset and base functions. In the representation in Fig. 2, the communication system is in a situation where: the communication node 2.1 operates as a base for the communication nodes 2.2 and 8 2.3; the communication node 2.2 operates only as a handset; the communication node 2.3 operates as a handset for the communication node 2.1 and as a base for the communication nodes 2.4 and 2.5; the communication node 2.5 operates only as a handset; and the communication node 2.4 operates as a handset for the communication 5 node 2.3 and as a base for the communication node 2.6. Thus, data can be relayed by a node in order to put or maintain in relationship at least two other nodes. There thus exists a communication channel 2.10 between the nodes 2.1 and 2.2, a communication channel 2.11 between the nodes 2.1 and 2.3, a communication channel 2.12 between the nodes 2.3 and 2.4, a communication channel 2.13 between the nodes 2.3 and 2.5, 10 and a communication channel 2.14 between the nodes 2.4 and 2.6. It is also possible to refer to communication links. Except for the communication node 2.1 of the highest hierarchical level, referred to as master, each other communication node is attached to a base, and may itself serve as a base. In this way a communication tree is obtained forming a chain of 15 communication nodes enabling all these communication nodes to have the same clock reference. The set formed by a communication node acting as a base and the communication nodes that are attached thereto is called group or hierarchical level. The communication nodes in each group can communicate with each other by means of the group base. Four elementary communication units 1.6 of the frame 1.1 20 are then necessary to implement such communication. Information can also be exchanged between the groups by hierarchically climbing and/or descending the communication tree via the group bases. In these two cases, each base acts as a relay between the communication nodes, either within the group for which it acts as a base or between the group in which it acts as a base and the one, if such exists, in which it 25 acts as a handset. In other words, the communication system is a hierarchical cellular radio network defining hierarchical levels each comprising a base and at least one handset, the system being adapted so that the communication nodes are enabled to communicate via the base of their hierarchical level. The transmission resource common to the network neighbourhood is then an 30 information transmission means alternative to the hierarchical climbs and descents of the communication tree. The transmission resource common to the network neighbourhood enables to avoid suffering congestions that may appear at the bases of the communication system, by providing each communication node with broadcasting means independent of the base to which it is attached.

The individuals being mobile, the transmission conditions between the communication nodes may change rapidly, for example through the appearance of a phenomenon of signal jamming or by fading of the signal through the appearance of an obstacle between the communication nodes. The communication channels 2.10, 5 2.11, 2.12, 2.13 and 2.14 may then suffer interference detrimental to communication, or even become unusable. For example, following a signal jamming, the communication channel 2.14 becomes unusable and the communication node 2.6, acting as a handset, can no longer synchronise with the communication node 2.4, acting as a base. It is then necessary 10 for the communication node 2.6 to synchronise with another communication node acting as a base. The handset can then use the information broadcasted via the transmission resource common to the network neighbourhood to accelerate its resynchronisation with a base. The use of the resource common to the network neighbourhood for 15 broadcasting such information is detailed hereafter in relation to Fig. 5. This thus prevents it from scanning the whole transmission spectrum in order to detect a base beacon on which it could attach. Such use of the information broadcasted via the transmission resource common to the network neighbourhood is described hereafter in relation to Fig. 7. 20 The handset may also use the resource common to the network neighbourhood to broadcast a discovery message, called hello message, to indicate to the other communication nodes that it is seeking a base to which to attach. The use of the resource common to the network neighbourhood for broadcasting such a message is detailed hereafter in relation to Fig. 5. 25 In addition, such fast fading or fast shadowing phenomena may be detected only by a subset of the communication nodes. Such a situation must be notified to the rest of the communication system, so that a redefinition of the communication channels and the base-handset roles can be performed as quickly as possible. The handset can then use the transmission resource common to the network 30 neighbourhood to broadcast information concerning the transmission conditions and thus accelerate its broadcasting in the communication system. Such use of the information broadcasted via the transmission resource common to the network neighbourhood is described hereafter in relation to Fig. 6.

Such a use of information broadcasted via the transmission resource common to the network neighbourhood is particularly useful when a phenomenon of jamming, fast shadowing or fast fading occurs between the handset and the base to which it is attached. 5 Such a use of the information broadcasted via the transmission resource common to the network neighbourhood is also particularly useful when the communication node in question acts as a base, this node is congested and a jamming, fast shadowing or fast fading occurs between the node and the base to which it is attached. This is because, in a particular embodiment, each communication node can 10 transmit or receive only on a single carrier frequency at each time slot. Thus, when a communication node acting as a base is highly stressed, it has no or little possibility of detecting the transmissions of beacons from the other communication nodes. If the communication with the base to which it is attached is broken, it will be necessary for it to parse through all the carrier frequencies seeking beacons, thus causing its 15 disconnection from the communication system during several TDMA frames. Fig. 3 schematically illustrates an architecture of a communication node in the communication system. This architecture comprises, linked by a communication bus 3.1: a processor, microprocessor, microcontroller (denoted kc) or CPU (Central Processing Unit) 3.2; a random access memory RAM 3.3; a read only memory ROM 20 3.4; a storage medium reader 3.5, such as an SD card (Secure Digital card) reader; radio interface means 3.6; and man-machine interface means 3.7. The microcontroller 3.2 is capable of executing instructions loaded into RAM 3.3 from ROM 3.4, an external memory (not shown), a storage medium, such as an SD card or the like, or a communications network. When the communication node is 25 powered up, the microcontroller 3.2 is capable of reading instructions from RAM 3.3 and executing them. These instructions form a computer program, which causes the implementation, by the microcontroller 3.2, of all or some of the algorithms described hereafter in relation to Figs. 4 to 7. All or some of the algorithms described hereafter in relation to Figs. 4 to 7 can 30 be implemented in software form through the execution of a set of instructions by a programmable machine, such as a DSP (Digital Signal Processor) or a microcontroller, or be implemented in hardware form by a machine or a dedicated component, such as an FPGA (Field-Programmable Gate Array) or an ASIC (Application-Specific Integrated Circuit).

Fig. 4 schematically illustrates an algorithm for setting up the transmission resource common to the network neighbourhood. The algorithm illustrated in Fig. 4 is implemented by a dedicated node in the communication system. It is preferentially implemented by the node of the highest 5 hierarchy, the master node, i.e. the communication node 2.1 in the representation in Fig. 2. The master node is the one, in the network neighbourhood, that acts only as a base, the other communication nodes acting either as a handset, or as a base and handset. In a variant embodiment, this master node may be elected among the nodes in the communication system, in particular to take into account the fact that, following 10 an appearance or disappearance of a phenomenon of jamming, fast shadowing or fast fading, the communication system may be caused to divide into two or, respectively, two communication systems may be caused to merge. In a step 4.1, the master node 2.1 determines the transmission conditions in the communication system. This step may be performed by analysing the signals received 15 by the master node 2.1 via its interface 3.6 and/or from information supplied by other nodes in the communication system. Such information may be supplied either in unicast mode by the communication nodes in the group of the master node, or in broadcast mode via the transmission resource common to the network neighbourhood previously allocated. When this information is supplied by the communication nodes 20 in the group of the master node, it may be information relayed by these nodes on behalf of groups of a lower hierarchical level. The master node 2.1 thus determines, in the frame 1.1, the elementary communication units 1.6 which, according to the transmission conditions, may be used by all the nodes in the communication system. 25 In a following step 4.2, the master node 2.1 selects the transmission resource common to the network neighbourhood, according to the transmission conditions determined in the step 4.1. The transmission resource common to the network neighbourhood may be a single elementary communication unit 1.6 or several elementary communication units 30 1.6, which may then be contiguous or separate in the representation in Fig. 1. In a following step 4.3, the master node 2.1 notifies to the other nodes in the communication system which elementary communication unit or units 1.6 are allocated to form the transmission resource common to the network neighbourhood. Preferably, this notification is made by the master node 2.1 in the beacon that it transmits at each frame 1.1. This notification is then relayed by each of the communication nodes in the group of the master node 2.1 and which also acts as a base for another group. This notification can then be relayed in these other groups by means of the beacons transmitted by the bases of these groups. The notification may 5 once again be relayed so as to be broadcasted in all the communication tree via the beacons sent by the bases of the groups. The communication nodes can thus be free of the hierarchical character of the communication tree and transmit messages and information without passing via the base to which it is attached, using the transmission resource common to the network 10 neighbourhood. All the communication nodes are thus allowed to access the transmission resource common to the network neighbourhood selected and notified by the master node 2.1. In a particular embodiment, the step 4.1 is performed for each frame 1.1 and a new transmission resource common to the network neighbourhood is selected, and 15 then notified, in the event of a change in transmission conditions meaning that at least one elementary communication unit 1.6 of the transmission resource common to the network neighbourhood previously allocated is no longer usable. Fig. 5 schematically illustrates a first algorithm for using the transmission resource common to the network neighbourhood. 20 According to a first example embodiment, the transmission resource common to the network neighbourhood is used to broadcast messages comprising synchronisation information and/or information concerning the topology of the network consisting of the communication nodes. In a step 5.1, the communication node in question generates such a message. This message, referred to as neighbourhood message, may 25 comprise: - a field comprising an identifier of the communication node in question; - a field comprising an identifier of the group to which the communication node in question belongs; - a field comprising an identifier of elementary communication unit 1.6 in 30 which the communication node in question transmits a beacon, when the communication node in question acts as a base; - a field comprising a list of identifiers of a predefined quantity of communication nodes that are the closest to the communication node in question, i.e. those for which the energy of the received signals is the highest; - a field comprising an identifier of the base to which the communication node in question is possibly attached; - a field comprising a signal transmission quality indicator, or a distance estimation indicator, between the communication node in question and the base to 5 which it is possibly attached; - a field comprising an indication of loading of the cell, when the communication node in question acts as a base. When a communication node receives such messages via the transmission resource common to the network neighbourhood, this enables it to be situated in a 10 communication system and to know its network neighbours, i.e. those with which direct communication can be set up. A use of these messages by a communication node for resynchronising itself is described hereafter in relation to Fig. 7. In a following step 5.2, the node in question determines what is the transmission resource common to the network neighbourhood in the frame 1.1. This information is 15 obtained following the notification transmitted at the step 4.3. In a following step 5.3, the communication node in question broadcasts, via the transmission resource common to the network neighbourhood, the message generated at the step 5.1. Broadcasting via the transmission network common to the network 20 neighbourhood can be performed in a predetermined manner according to a sequence fixed in advance or in a random or pseudo-random manner. According to a second example embodiment, the transmission resource common to the network neighbourhood is used by the communication node in question to broadcast a discovery message, referred to as hello message, to indicate to the other 25 communication nodes that it is seeking a base to which to attach. This communication node in question may then receive in response, via the transmission resource common to the network neighbourhood, at least one neighbourhood message, as previously mentioned, broadcasted by the node or nodes in the network neighbourhood that received the hello message. 30 Other topology control messages can thus be transmitted according to one or other or both of these example embodiments. Fig. 6 schematically illustrates a second algorithm for using the transmission resource common to the network neighbourhood. The transmission resource common to the network neighbourhood is then used to broadcast information with regard to the transmission conditions and thus to accelerate the broadcasting thereof in the communication system. In a step 6.1, the communication node in question detects a change in transmission conditions, or at least determines these conditions. In particular when 5 these transmission conditions may cause the loss of at least one transmission channel, the communication node in question determines what is the transmission resource common to the network neighbourhood in the frame 1.1, in a following step 6.2. This information is obtained following the notification transmitted at the step 4.3. In a following step 6.3, the communication node in question broadcasts, via the 10 transmission resource common to the network neighbourhood, a message comprising information representative of the transmission conditions, or changes in transmission conditions, determined at the step 6.1. Such a broadcast mode via the transmission resource common to the network neighbourhood is particularly effective when there is a loss of the communication 15 channel between the communication node in question and the base to which it is attached, or between the communication node in question and the handset or handsets attached thereto. When the changes in transmission conditions concern a degradation of the transmission via the common resource of the network neighbourhood, the 20 communication node in question transmits the message towards the master node 2.1 using a climbing transfer from base to base on the communication tree. When the changes in transmission conditions concern a degradation of the transmissions via a part of the resource common to the network neighbourhood, the communication node in question transmits the message via the other part of the 25 resource common to the network neighbourhood. The method in Fig. 6 may also be used by a base to indicate to the nodes in the network neighbourhood that it is congested. Fig. 7 schematically illustrates a resynchronisation algorithm using information broadcasted via the transmission resource common to the network neighbourhood. 30 In a step 7.1, the communication node in question detects a loss of communication channel with the base to which it is attached. In a following step 7.2, the communication node in question obtains information broadcasted via the transmission resource common to the network neighbourhood and which is aimed at enabling performing a resynchronisation with another base. This information is that contained in the neighbourhood messages previously mentioned in relation to Fig. 5. As already mentioned, these messages can be transmitted periodically by the communication nodes or be transmitted in response to a hello message transmitted by the communication node in question. 5 In a following step 7.3, the communication node in question uses the information contained in the received neighbourhood messages in order to set up a communication channel with another base. Fig. 8 schematically illustrates an algorithm for improving connectivity of the network neighbourhood using information broadcasted via the transmission resource 10 common to the network neighbourhood. In a step 8.1, the communication node in question obtains information broadcasted via the transmission resource common to the network neighbourhood and which enables obtaining a description of the topology of the network formed by the communication nodes. This information is that contained in the neighbourhood 15 messages previously mentioned in relation to Fig. 5. In a variant embodiment, the communication node in question receives from another communication node a message comprising information on change in transmission conditions, as indicated previously in relation to Fig. 6. In a following step 8.2, the communication node in question detects a loss of 20 connectivity with a communication node, meaning that the messages received by the communication node in question show that the communication node that transmitted them detected a drop in communication quality liable to cause a loss of at least one communication channel. In a following step 8.3, the communication node in question checks whether it 25 already has the role of a base and, if such is not the case, it takes the role of a base and activates a beacon accordingly. In a following step 8.4, the communication node in question broadcasts, via the transmission resource common to the network neighbourhood, a neighbourhood message, as presented previously in relation to Fig. 5, indicating the activation of this 30 beacon and supplying the information enabling synchronisation. In a particular embodiment, apart from the use of a transmission resource common to the network neighbourhood, each base can use a transmission resource common to its cell and enable the handsets of its cell to access this resource.

The transmission resource common to a cell is preferentially an elementary communication unit 1.6 that is the complement in the second part 1.3 of the one used in the first part 1.2 of the frame 1.1 for the base for transmitting the beacon signals. In the representation in Fig. 1, for beacon signals transmitted over a given carrier 5 frequency in a time slot N, the transmission resource common to the cell corresponds to the time slot N+12 for this same carrier frequency. This common transmission resource concerns the handsets of the cell and has a predefined position relative to a resource used by the base for transmitting a beacon signal. Its access is, for the handsets in the cell, similar to that of the resource common to the network 10 neighbourhood. It may also be sequentially predefined in time. The transmission resource common to a cell may be used by a communication node in this cell in order to broadcast a neighbourhood message, as presented previously in relation to Fig. 5. The communication node acting as a base for this cell may relay the information transmitted via the transmission resource common to the 15 other nodes in the cell. The communication node acting as a base for the cell may relay the information transmitted via the common transmission resource to the higher hierarchical level. This is for example the case when the transmission resource common to the cell is used to broadcast information relating to the transmission conditions or changes in transmission conditions. 20 The communication node acting as a base may detect a jamming, fast shadowing or fast fading situation, by means of the neighbourhood messages transmitted by the handset in its cell, and thus decide to change the elementary communication unit 1.6 used for transmitting its beacon. The same method can be implemented using neighbourhood messages transmitted by the handsets of this cell via the transmission 25 resource common to the network neighbourhood. In a particular embodiment, the bases of the communication system are adapted not to transmit their beacon in the same time slot or slots as the transmission resource common to the network neighbourhood. When a base changes the elementary communication unit 1.6 used for transmitting its beacon, it preferentially ensures that 30 it selects a time slot different from the one or ones of the transmission resource common to the network neighbourhood. When the master node changes the elementary communication unit 1.6 defining the transmission resource common to the network neighbourhood, it preferentially ensures that it selects a time slot different from those used by the bases to transmit their beacons. Such information can be transferred to the master node by climbing the hierarchical tree.

Claims

1. Communication system comprising communication nodes (2.1; 2.2; 2.3; 2.4;

2.5; 2.6) communicating by means of frames (1.1) of time division and frequency 5 division multiple access type, said nodes forcing a hierarchical cellular radio network defining hierarchical levels each comprising a base (2.3) and at least one handset (2.4; 2.5), said system being adapted so that the communication nodes are allowed to communicate via the base of their hierarchical level, characterised in that a communication node (2.1), referred to as master node, comprises: 10 - means for selecting (4.2), in each of said frames, a transmission resource common to said communication nodes, referred to as transmission resource common to the network neighbourhood; - means for broadcasting (4.3) to said communication nodes, through the hierarchical levels via their respective bases, information representative of the selected 15 common transmission resource; and in that said system is adapted to allow said communication nodes to broadcast messages via the selected common transmission resource. 2. Communication system according to claim 1, characterised in that said master 20 node comprises means for obtaining (4.1) information representative of transmission conditions between said communication nodes and in that the means for selecting the transmission resource common to the network neighbourhood are adapted to perform the selection according to said obtained information. 25

3. Communication system according to claim 3, characterised in that said communication nodes comprise means for determining (6.1) transmission conditions in said communication system and means for transmitting (6.3) messages comprising information representative of transmission conditions, or changes in transmission conditions, either via the transmission resource common to the network 30 neighbourhood or via the base of their hierarchical level.

4. Communication system according to any one of claims 1 to 3, characterised in that said communication nodes comprise means for broadcasting (5.3), via the transmission resource common to the network neighbourhood, messages comprising 19 synchronisation information identifying a resource of said frames used for transmitting a beacon signal.

5. Communication system according to any one of claims 1 to 4, characterised in 5 that, said communication nodes comprising handset and base functions, said communication nodes comprise: - means for detecting (8.2) a risk of loss of connectivity with a communication node, - means for activating (8.3) a beacon signal, when a risk of loss of connectivity 10 is detected; - means for broadcasting (8.4), via the transmission resource common to the network neighbourhood, a message comprising synchronisation information identifying a resource of said frames, used for transmitting said beacon signal. 15

6. Communication system according to any one of claims I to 5, characterised in that the base of each hierarchical level comprises means for enabling the communication nodes of its hierarchical level to broadcast messages via a transmission resource common to said hierarchical level. 20

7. Communication system according to claim 6, characterised in that the transmission resource common to each hierarchical level corresponds to a resource of said frames having a predefined position relative to a resource used by the base of said hierarchical level to transmit a beacon signal. 25

8. Communication system according to either one of claims 6 and 7, characterised in that the base of each hierarchical level comprises means for changing resource of said frames, used for transmitting a beacon signal, according to messages received via the transmission resource common to its hierarchical level or via the transmission resource common to the network neighbourhood. 30

9. Communication system according to any one of claims 6 to 8, characterised in that said system is adapted so that the bases transmit their beacon signal in a time slot distinct from each time slot defining the transmission resource common to the network neighbourhood. 20

10. Communication method in a communication system comprising communication nodes (2.1; 2.2; 2.3; 2.4; 2.5; 2.6) communicating by means of frames (1.1) of time division multiple access type, said nodes forming a hierarchical cellular 5 radio network defining hierarchical levels each comprising a base (2.3) and at least one handset (2.4; 2.5), said system being adapted so that the communication nodes are enabled to communicate via the base of their hierarchical level, characterised in that a communication node (2.1), referred to as master node, performs steps consisting of: - selecting (4.2) in each of said frames a transmission resource common to 10 said communication nodes, referred to as transmission resource common to the network neighbourhood; - broadcasting (4.3) to said communication nodes, through the hierarchical levels via their respective bases, information representative of the selected common transmission resource; 15 and in that said method is such that said communication nodes are allowed to broadcast messages via the selected common transmission resource.

11. Computer program, characterised in that it comprises instructions for implementing, by means of a communication node, the method according to claim 10, 20 when the program is executed by a processor (3.2) of said communication node.

12. Storage means, characterised in that they store a computer program comprising instructions for implementing, by means of a communication node, the method according to claim 10, when said program is executed by a processor (3.2) of 25 said communication node.